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Ellesmeroceratidae
The Ellesmeroceratidae constitute a family within the cephalopod order Ellesmerocerida, characterized by straight and endogastric shells, often laterally compressed, so the dorso-ventral dimension is slightly greater than the latteral., with close spaced sutures having shallow lateral lobes and a generally large tubular ventro-marginal siphuncle with concave segments and irregularly spaced diaphragms. Connecting rings are thick and layered, externally straight but thickening inwardly with the maximum near the middle of the segment so as to leave concave depressions on internal siphuncle molds. Septal necks are typically orthochoanitic but vary in length from almost absent (achoanitic) to reaching half way to the previous septum (hemichoanitic) and may even slope inwardly (loxochoanitic).(Flower 1964) Evolution and Phylogeny The Ellesmeroceratidae have their derivation in the Plectronoceratidae, order Plectronocerida, in Trempealeauan stage of the Late Cambrian from which time 13 genera have been described (Chen and Teichert 1983, Teichert 1988). The earliest described, assinged to the Ellesmeroceratidae, is the early Trempealeauan Hunuanoceras which comes from the lower part of the upper Yenchou Member of the Fengshan Formation in China. Hunuanoceras is a small endogastric cyrtocone resembling the ascestral Plectronoceras except for having resistant calcified connecting rings. Hunuanoceras is followed by Eburoceras which first appears in the upper part of the upper Yenchou and continues thoughout the overlying Wanwankou Member of the Fengshan. The Wanwankou is middle and early upper Trempealeauan. The remaining eleven genera are restricted to the Wanwankou, except for Clarkoceras and Ectenolites which persist into the Lower Ordovician. (Chen and Teichert 1983). Clarkoceras and Ectenolites provide the ancestry for the diverse ellesmerceratitds of the Early Ordovician, Gasconadian, and those which followed. The Gasconadian was dominated to virtual exclusion by the Ellesmeroceratidae which diversified during that time into a variety of forms and genera. (Flower 1964). Some like Ellesmeroceras and Eremoceras were straight shelled, following the example of Ectenolites. Others like Dakeoceras and Burenoceras were endogastric in the sense of Clarkoceras. Gradations are found between elongate (longiconic) and short (breviconic) forms and between straight (orthoconic) and curved (cyrtoconic) forms and between those with simple open apertures and those in which the aperture is contracted The Ellesmeroceratidae gave rise within the Ellesmerocerida to the Protocycloceratidae, Bassleroceratidae, and possibly the Cylostomiceratidae in the early Canadian (late Gasconadian (?), Demingian) and to the Bathmoceratidae, and Cyrtocerinidae in the late Canadian (late Jeffersonian or Cassinian). The Ellesmeroceratidae also gave rise at about the close of the Gasconadian to the Endocerida, Tarphycerida, and to the Orthocerida through the ancestral Baltoceratidae. (Teichert 1964, Furnish and Glenister 1964, Flower 1976, Kroger and Mutvie 2005), at which time they cease to be the dominant element in cephalopod faunas References *Chen and Teichert 1983; Cambrain Cephalopods, Geology V.11, pp 647–650, Nov 1983 *Flower 1964; The Nautiloid Ordre Ellesmerocerida (Cephalopoda) Menoir 12, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM *Flower 1976; Ordovician Cephalopod Faunas and Their Role in Correlation, pp 531–537 in The Ordovician System: Proceedings of a Paleontological Association Symposium; Univ of Wales and Welsh Nat’l Mus Press. *Furnish and Glenister 1964; Nautiloidea - Tarphycerida; In the Treatise on Invertebrate Paleontology Part K Nautiloidea. Curt Teichert and R.C.Moore, eds, Geological Society of America and Univ Kansas Press *Kroger and Mutvie 2005; Nautiloids with multiple paired muscle scars from Early-Middle Ordovician of Baltoscandia. - Its bearing for taxonomic and phylogenetic reconstructions; Palaeontology, 48:1-11. *Teichert 1964; Endoceratoidea, in the Treatise on Invertebrate Paleontology, Part K Nautiloidea. Curt Teichert and R.C.Moore, eds, Geological Society of America and Univ Kansas Press *Teichert 1988; Main Features of Cephalopod Evolution, in The Mollusca, M.R Clarke and E.R.Trueman, eds; Academic Press. Category:Nautiloids